There is no option for changing the font style in current version of Qliksense, but you can do this using a extension. Better you search for ViZLib extension in google and if you have permission to import an extension through QMC, you can try that extensions.

Out the box you can for Kent the appearance of the measure values and that's it. you can use themes to change the appearance of an overall app, fonts, colours, backgrounds. look at the high viz example for what can be done.

Specific chart header formatting would only be available if someone has written an extension for it. I hear ones by climber are good but don't know if they have covered your use case.